The majority of vertebrate cell cultures in vitro are grown as monolayers on an artificial substrate bathed in nutrient medium. The nature of the substrate on which the monolayers grow may be solid, such as plastic, or semisolid gels, such as collagen or agar. Disposable plastics have become the preferred substrate used in modern-day tissue or cell culture.
A few researchers have explored the use of natural substrates related to basement membrane components. Basement membranes comprise a mixture of glycoproteins and proteoglycans that surround most cells in vivo. For example, Reid and Rojkund (1979, In, Methods in Enzymology, Vol. 57, Cell Culture, Jakoby & Pasten, eds., New York, Acad. Press, pp.263-278); Vlodavsky et al., (1980, Cell 19:607-617); Yang et al., (1979, Proc. Natl. Acad. Sci. USA 76:3401) have used collagen for culturing heptocytes, epithelial cells and endothelial tissue. Growth of cells on floating collagen (Michalopoulos and Pitot, 1975, Fed. Proc. 34:826) and cellulose nitrate membranes (Savage and Bonney, 1978, Exp. Cell Res. 114:307-315) have been used in attempts to promote terminal differentiation. However, prolonged cellular regeneration and the culture of such tissues in such systems has not heretofore been achieved.
Cultures of mouse embryo fibroblasts have been used to enhance growth of cells, particularly at low densities. This effect is thought to be due partly to supplementation of the medium but may also be due to conditioning of the substrate by cell products In these systems, feeder layers of fibroblasts are grown as confluent monolayers which make the surface suitable for attachment of other cells. For example, the growth of glioma on confluent feeder layers of normal fetal intestine has been reported (Lindsay, 1979, Nature 228:80).
While the growth of cells in two dimensions is a convenient method for preparing, observing and studying cells in culture, allowing a high rate of cell proliferation, it lacks the cell-cell and cell-matrix interactions characteristic of whole tissue in vivo. In order to study such functional and morphological interactions, a few investigators have explored the use of three-dimensional substrates such as collagen gel (Douglas et al., 1980, In Vitro 16:306-312; Yang et al., 1979, Proc. Natl. Acad. Sci. 76:3401; Yang et al., 1980, Proc. Natl. Acad. Sci. 77:2088-2092; Yang et al., 1981, Cancer Res. 41:1021-1027); cellulose sponge, alone (Leighton et al., 1951, J. Natl. Cancer Inst. 12:545-561) or collagen coated (Leighton et al., 1968, Cancer Res. 28:286-296); a gelatin sponge, Gelfoam (Sorour et al., 1975, J. Neurosurg. 43:742-749).
In general, these three-dimensional substrates are inoculated with the cells to be cultured. Many of the cell types have been reported to penetrate the matrix and establish a "tissue-like" histology. For example, three dimensional collagen gels have been utilized to culture breast epithelium (Yang et al., 1981, Cancer Res. 41:1021-1027) and sympathetic neurons (Ebendal, 1976, Exp. Cell Res. 98:159-169). Additionally, various attempts have been made to regenerate tissue-like architecture from dispersed monolayer cultures. Kruse and Miedema (1965, J. Cell Biol. 27:273) reported that perfused monolayers could grow to more than ten cells deep and organoid structures can develop in multilayered cultures if kept supplied with appropriate medium (see also Schneider et al., 1963, Exp. Cell Res. 30:449-459 and Bell et al., 1979, Proc. Natl. Acad. Sci. USA 76:1274-1279); Green (1978, Science 200:1385-1388) has reported that human epidermal kerotinocytes may form dematoglyphs (friction ridges) if kept for several weeks without transfer; Folkman and Haudenschild (1980, Nature 288:551-556) reported the formation of capillary tubules in cultures of vascular endothelial cells cultured in the presence of endothelial growth factor and medium conditioned by tumor cells; and Sirica et al. (1979, Proc. Natl. Acad. Sci U.S.A. 76:283-287; 1980, Cancer Res. 40:3259-3267) maintained hepatocytes in primary culture for about 10-13 days on nylon meshes coated with a thin layer of collagen. However, the long term culture and proliferation of cells in such systems has not been achieved.
Indeed, the establishment of long term culture of tissues such as bone marrow has been attempted. Overall the results were disappointing, in that although a stromal cell layer containing different cell types is rapidly formed, significant hematopoiesis could not be maintained for any real time. (For review see Dexter et al., In Long Term Bone Marrow Culture, 1984, Alan R. Liss, Inc., pp.57-96).